1. Field of the Invention
The present invention relates generally to planographic printing, and in particular to an apparatus for continuously supplying new plate material to the plate cylinder of planographic printing press or plate-imaging apparatus.
2. Description of the Related Art
U.S. Pat. No. 5,355,795 (the entire disclosure of which is hereby incorporated by reference) describes a system providing for the continuous provision of blank lithographic plate material around a plate cylinder for automated imaging and subsequent printing therewith. In the disclosed system, a feeder spool installed within the cylinder contains a rolled supply of plate material, which wraps around the cylinder and is received by an uptake spool, also located within the cylinder. The assembly maintains a strong tension along the wrapped material, and is driven by the same power source used to rotate the plate cylinder; upon actuation by a user the system couples rotary power to the spools, drawing fresh plate material from the supply spool and advancing used plate material then surrounding the cylinder to the uptake spool.
The '795 patent envisions installation of the supply and uptake spools within a cassette frame that is inserted into the cylinder body and is withdrawn when all plate material has been used. This arrangement ensures that the spools, when introduced into the cylinder, will be precisely aligned with complementary components of the plate-advancing system; obviously, substantially perfect alignment is required for proper cooperation among the components.
The cassette arrangement, however, has certain limitations. Chief among these are cost and weight. The relatively heavy frame construction of the cassette, when disposed within the cylinder, demands additional rotary power to turn the complete assembly. More importantly, the extra weight can cause an out-of-balance condition unless all components are carefully arranged within the cylinder, imposing stringent spatial relationships among the interior cylinder components and limiting design options. The cost associated with the cassette frame is multiplied not only by the number of print stations on a press utilizing the plate-advancing system, but also by the typical need to maintain at least one pre-loaded cassette outside the press for each such cylinder in order to avoid idle press time.